1. Field of the Invention
The present invention relates to a novel polyimide and a process for preparing the polyimide, and a novel aromatic diamino compound used for preparing polyimide and a process for preparing the aromatic diamino compound. More particularly the invention relates to thermoplastic polyimide having excellent heat resistance and a preparation process thereof and further relates to a polyimide based resin composition comprising said polyimide and a fibrous reinforcement.
A novel aromatic diamino compound of the formula (3-1) having a pyridine skeleton and a novel aromatic diamino compound represented by the formula (3-2) having a cyano radical are useful as the raw material of polyimide in the invention and can also be applied to the raw material of polyamide, polyimide, polyamideimide, bismaleimide and epoxy resin.
2. Related Art of the Invention
Conventionally, polyimide has been widely used in the field of molding materials, composite materials and electric.electronic appliances because polyimide has excellent heat resistance and additionally is excellent in mechanical properties, chemical resistance, flame retardance, electrical properties and other characteristics.
For example, a representative polyimide which has been known is Kapton and Vespel (Trade Mark of E. I. Du Pont de Nemours & Co.) which have recurring structural units of the formula (A): ##STR3##
The polyimide, however, is insoluble and infusible and thus must be molded by using a specific technique such as sinter molding by way of polyamic acid precursor. Such problem leads to difficulty in processing and the process is difficult to provide articles having a complex shape. Consequently, finishing operation such as cutting is additionally required in order to obtain satisfactory articles and causes a serious defect of high processing cost.
Similarly, another type of polyimide which has been generally known as a film forming resin is Upilex (Trade Mark of Ube Industries Co.) which has recurring structural units represented by the formula (B): ##STR4## wherein Ar is ##STR5##
The resin itself has no melt-flowability and cannot be extruded. Consequently, the resin has a problem that the resin film can be obtained by casting alone.
Japanese Laid-Open Patent SHO 62-205124 has disclosed thermoplastic polyimide which has improved glass transition temperature, melt-flowability and processability and has recurring structural units of the formula (C): ##STR6##
The polyimide has good melt-flowability and can be processed by injection molding. The polyimide, however, is essentially crystalline and thus crystallization proceeds with heat treatment under specific conditions. When once crystallized, processing temperature above the melting point of the polymer (390.degree. C. or more in the case of the polyimide/is required for melting the resin again. Softness, that is, flexibility of polymer itself is an important factor for film utilities of polyimide and other polymers.
A crystallized film has insufficient flexibility and is liable to cause fracture or micro-cracks by external force.
From the viewpoint of the above problems, an essentially amorphous and thermoplastic polyimide resin which leads to no crystallization after melt-processing and has flexibility is desired in the field of use which requires substantially no crystallization.
In order to improve such defect of polyimide, a process for improving the diamine component of materials has been tried. For example, a process for controlling glass transition temperature and melt-flowability of polyimide has been carried out by exchange of a bonding radical in a monomer unit, extension of a polymer chain or formation of a bent structure.
For example, as to the bent structure of the polymer chain, polyimide derived from 3,3'-diaminobenzophenone and 3,3',4,4'-benzophenonetetracarboxylic dianhydride has been developed by NASA as a thermoplastic polyimide LARC-TPI.
The polyimide, however, is still insufficient in melt-flowability, though excellent in heat resistance and adhesion. Consequently, the polyimide is now used primarily as an adhesive.
Various structures of polyimide has also been proposed concerning the process for extending the polymer chain. However, the structure which satisfies the whole conditions has not yet been obtained because of reduction in physical properties or other reasons.
Polyimide having a pyridine skeleton has been examined in the investigation of amino compounds as a monomer of such polyimide. Japanese Laid-Open Patent SHO 62-116563 has disclosed polyimide obtained by using bis(aminophenoxy)pyridine as a diamine component. However, polyimide prepared by using the compound as a monomer is affected by a short length of monomer units in the polymer chain and leads to insufficient melt-flowability and difficult processing.
Concerning polyimide having a cyano radical, Japanese Laid-Open Patent HEI 3-17129 has disclosed polyimide prepared from bis(aminophenoxy)benzonitrile as a monomer. Polyimide having a cyano radical in the polymer chain exhibits high heat resistance. However, the short length of the monomer units in the polymer chain leads to insufficient melt-flowability and difficulty in processing.
Polyimide which has nitrogen atoms in the polymer chain and is prepared by using diamine of a long molecular chain as a monomer has not yet been fully known on the properties such as heat resistance, processability, mechanical feature and crystallinity.